While machining/grinding is performed using a machine tool, if wear or chipping occurs in the tool, the machining precision for the workpiece deteriorates, producing defective workpieces. If the yield rate decreases due to the occurrence of defective workpieces, a large amount of loss may occur depending on the cost of the workpieces. For this reason, techniques for diagnosing abnormalities in tools have been proposed in which various signals are measured in real time during machining and compared to thresholds for the signals (Patent Documents 1 to 3).